Optical element

ABSTRACT

An optical element is disclosed. In an embodiment an optical element includes a substrate having a silicone surface, an antireflection layer overlying the silicone surface, wherein the antireflection layer comprises a first organic layer having a reflection-reducing nanostructure, the nanostructure having a depth of at least 30 nm, and a cover layer overlying the first organic layer, the cover layer having a thickness of no more than 40 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.14/693,627, filed Apr. 22, 2015, which claims the priority of Germanpatent application 10 2014 105 939.5, filed Apr. 28, 2014, each of whichis incorporated herein by reference in their entirety.

TECHNICAL FIELD

The invention relates to a method for producing an antireflection layeron a silicone surface and to an optical element having a siliconesurface and an antireflection layer applied thereon.

BACKGROUND

Silicones are synthetic poly(organo)siloxanes in which silicon atoms arelinked via oxygen atoms (inorganic polymers). Silicones have alreadybeen used industrially for some decades. Only in recent years, however,have silicones gained increasing importance for optical uses. Primarilyin automobile manufacture, in the field of LED lighting technology andfor optics for solar applications, highly transparent materials whichmaintain their properties for a long time even at elevated temperature(>100° C.) are required. Silicones differ fundamentally from the classof thermoplastics and also from the class of inorganic glasses. Asinorganic polymers, they form solid phases far above the glasstransition point, while both thermoplastics and inorganic glasses aregeometrically stable only below the glass transition temperature. Thisfact also leads to the very high thermal stability of silicones whilemaintaining the glass-clear optical impression.

Recently, there have been new types of highly transparent liquidsilicones on the market, which inter alia can be processed by specialinjection-molding processes to form high-precision optical parts.

One central theme of optical applications is antireflection, sincewithout antireflection several percent of the incident light is lost byreflection at any surface in the optical system. All silicones exhibit acertain elastic behavior and relatively high thermal expansion.Permanent bonding to rigid brittle layers, such as conventional opticalinterference layers made of oxides constitute, is therefore difficult toachieve. Brittle oxide layers fracture and form cracks as soon as thesubstrate deforms. Furthermore, low molecular weight compounds are oftenalso released from silicone components. These make durable bonding ofcoatings more difficult. Furthermore, silicone surfaces are generallyvery nonpolar, and therefore cannot be wetted and coated withoutactivation.

There are therefore no examples in which silicones have been coated in avacuum with conventional interference layer systems for the purpose ofantireflection.

German patent document DE 10241708 B4 (U.S. counterpart applicationpublication US 2005/0233083) and European patent document EP 2083991 B1(U.S. counterpart application publication US 2009/0261063) describemethods with which a nanostructure, by which the reflection of theplastic substrate is reduced, is produced on the surface of a plasticsubstrate by means of a plasma etching process. Such a plasma etchingprocess, however, is not suitable for the etching of silicone surfaces.

SUMMARY

Embodiments of the invention provide a method with which a thermallystable antireflection layer, which leads to antireflection in a largerange of wavelengths and angles of incidence, can be produced on asilicone surface. An optical element having such an antireflection layeris furthermore provided.

According to at least one embodiment, the method for producing anantireflection layer on a silicone surface comprises the application ofan organic layer and the production of a reflection-reducingnanostructure in the organic layer by a plasma etching process. In themethod, the reflection-reducing nanostructure is advantageously producednot in the silicone surface but in an organic layer applied thereon.

The organic layer advantageously comprises an organic material which isdistinguished by thermal stability up to a temperature of at least 150°C., preferably at least 200° C. Particularly preferably, the organiclayer contains melamine (2,4,6-triamino-1,3,5-triazine), MBP(5,5′-di(4-biphenylyl)-2,2′-bithiophene), TPD(N,N′-bis(3-methylphenyl)-N,N′-diphenyl-benzidine), NPB(N,N-di(naphth-1-yl)-N,N′-diphenyl-benzidine), TPB(N,N,N′,N′-tetraphenylbenzidine), TCTA(tris(4-carbazoyl-9-ylphenyl)amine), B2TP(5,5′-di-(4-byphenylyl)-2,2′-bithiophene), parylene.

The organic layer is preferably applied by a vacuum process, forexample, by thermal evaporation. The organic layer preferably has athickness of from 150 nm to 300 nm.

After the production of the nanostructure in the organic layer, a coverlayer is advantageously applied in a further method step. The coverlayer is preferably no more than 40 nm thick. Particularly advantageousis a thickness between 10 nm and 40 nm. In this range, the cover layeris on the one hand thick enough to ensure protection of thenanostructure, but on the other hand still thin enough not to impair, orto impair insubstantially, the reflection-reducing effect of thenanostructure.

The cover layer preferably contains a silicon oxide or an aluminumoxide. As an alternative, the cover layer may be formed from a plasmapolymer which comprises siloxane monomers such as, for example, TMS(tetramethylsilane) or HMDSO (hexamethyldisiloxane).

It has been found that, by the application of an organic layer in whicha nanostructure can be produced by a plasma etching process, and thesubsequent application of a cover layer, a good antireflection effectcan be achieved for a silicone surface over a large range of angles andwavelengths. Furthermore, the antireflection layer is distinguished byhigh thermal stability, so that the silicone surface provided with theantireflection layer can be used continuously at a temperature of morethan 100° C. or even more than 150° C. The antireflection layer istherefore advantageous, in particular, for optical elements made ofsilicone which are intended to be used continuously at high operatingtemperatures.

In a preferred configuration of the method, the silicone surface ispretreated by ion bombardment before the application of the organiclayer. Preferably, the pretreatment by ion bombardment is carried outwith argon ions and/or oxygen ions, ion energies in the range of from 60eV to 160 eV preferably being used. The treatment time is advantageouslybetween 60 seconds and 600 seconds. By the pretreatment by means of ionbombardment, in particular the adhesion of the subsequently appliedorganic layer on the silicone surface is improved. As an alternative orin addition, in the method an adhesion layer may be applied onto thesilicone surface. The adhesion layer preferably has a thickness of nomore than 50 nm, particularly preferably between 5 nm and 15 nm. In apreferred configuration, the adhesion layer contains a silicon oxide,for example, SiO₂, SiO or SiO_(x). The adhesion layer may, for example,be produced by thermal evaporation or by plasma polymerization fromsiloxane monomers.

The nanostructure produced in the organic layer preferably has a depthof at least 30 nm. In other words, the nanostructure extends from thesurface of the organic layer at least 30 nm deep into the organic layer.The effect achieved by such a nanostructure is, in particular, that thenanostructured organic layer has a lower effective refractive index thana homogeneous layer of the material of the organic layer. In particular,the nanostructured organic layer has an effective refractive index whichis lower than the refractive index of silicone.

In an advantageous configuration of the method, after the production ofthe nanostructure, a second organic layer is applied onto thenanostructure, a second nanostructure being produced in the secondorganic layer by means of a second plasma etching process.

In a preferred configuration of the method, an etch stop layer isapplied onto the nanostructure before the application of the secondorganic layer. The etch stop layer advantageously prevents parts of thenanostructure from being removed when the second plasma etching processis carried out. The etch stop layer preferably has a thickness of nomore than 30 nm. A small thickness of this type for the etch stop layerhas the advantage that the effective refractive index in the interfaceregion between the nanostructure and the second nanostructure isinfluenced only insubstantially. The etch stop layer may, for example,be an SiO₂ layer.

The second organic layer preferably has a higher etching rate in thesecond plasma etching process than the previously applied organic layerunder the same process conditions. A nanostructure, by which an evenlower effective refractive index is produced in the second organic layerthan in the first organic layer, is advantageously produced in thesecond organic layer by the second plasma etching process. In this way,a particularly good antireflection effect is achieved on the siliconesurface.

The cover layer applied after the production of the nanostructure, oroptionally after the production of the second nanostructure, preferablyhas a thickness of no more than 40 nanometers. The cover layer may, inparticular, have the function of a protective layer for the at least onenanostructured organic layer. Furthermore, the cover layer mayadvantageously be an antifog layer or a hydrophobic layer. For example,an at least 10 nm thick cover layer of SiO₂ imparts permanent antifogproperties to the surface. A hydrophobic surface may, in particular, beachieved by the application of a polymer layer containing fluorine. Inthis configuration, the polymer layer containing fluorine is preferablybetween 5 nm and 10 nm thick. A hydrophobic surface has the advantagethat water can roll off the surface easily.

The method is suitable, in particular, for providing an optical elementmade of a silicone with an antireflection layer. The optical elementrendered antireflective by the method has a silicone surface and anantireflection layer applied onto the silicone surface, wherein theantireflection layer comprises an organic layer which comprises ananostructure having a depth of at least 30 nm, and a cover layer,following on from the organic layer, which has a thickness of no morethan 40 nm. Further advantageous configurations of the optical elementmay be found in the preceding description of the method, and vice versa.

The optical element is distinguished, in particular, in that theresidual reflection is very low over a large range of wavelengths andangles, and the optical element can be used continuously at hightemperatures of more than 100° C. or even more than 150° C. The siliconesurface of the optical element may, in particular, be a curved surfaceor a microstructured surface without the reflection-reducing effect ofthe antireflection layer being substantially compromised.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with the aid ofexemplary embodiments in connection with FIGS. 1 to 4, in which:

FIGS. 1A to 1E show a schematic representation of a method for producingan antireflection layer according to a first exemplary embodiment withthe aid of intermediate steps;

FIG. 2 shows the reflection R as a function of the wavelength λ of anoptical element having an antireflection layer according to oneexemplary embodiment in comparison with an uncoated silicone surface;

FIGS. 3A to 3F show a schematic representation of a method for producingan antireflection layer according to a further exemplary embodiment withthe aid of intermediate steps; and

FIG. 4 shows the reflection R as a function of the wavelength λ for anoptical element having an antireflection layer according to oneexemplary embodiment in comparison with an uncoated silicone surface forangles of incidence of 0° and 45°.

Components which are the same or have the same effect are respectivelyprovided with the same references in the figures. The componentsrepresented and the size proportions of the components with respect toone another are not to be regarded as true to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the method represented in FIGS. 1A to 1E according to one exemplaryembodiment, a silicone surface 7 is provided with an antireflectionlayer. The silicone surface 7 is the surface of a substrate 10 whichcomprises or consists of a silicone. The silicone may, in particular,comprise siloxane units of the general formula R₃Si—[O—SiR₂]n-O—SiR₃,where R may be hydrogen atoms or alkyl groups. The silicone may, forexample, be a molding silicone which is UV-curing, thermally curing oraddition-crosslinking at room temperature. Examples of such siliconesare poly(dimethylsiloxane) (PDMS) or the silicones available under thedesignations Wacker SilGel, Wacker SEMICOSIL, Bluesil 2-componentmolding compound, RTV 615 or Sylgard 184. Silicones which are intendedfor processing by injection molding are furthermore suitable. Suchsilicones are, for example, available under the following designations:Dow Corning MS1001, M1002, M1003, Wacker LUMISIL (for example, LUMISIL419 UV, LUMISIL LR 7600).

The substrate 10 may, in particular, be an optical element which may,for example, be intended for use in lighting technology, solartechnology, automobile technology or medical technology. Unlike in theexemplary embodiment represented, it is possible in particular for theoptical element 10 to have a curved surface and/or a microstructuredsurface.

As represented in FIG. 1A, in a first intermediate step a pretreatmentof the silicone surface 7 is advantageously carried out by ionbombardment, which is indicated by the arrows 5. The ion bombardment ispreferably carried out with argon ions and/or oxygen ions which have ionenergies in a range of from 60 eV to 160 eV. The silicone surface 7 ispreferably pretreated by ion bombardment during a treatment time ofbetween 60 seconds and 600 seconds.

In a further intermediate step of the method, represented in FIG. 1B, anadhesion layer 6 is advantageously applied onto the silicone surface 7.The adhesion layer 6 is advantageously no more than 50 nm thick,preferably between 5 nm and 15 nm thick. A silicon oxide layer, forexample, SiO₂, SiO or SiO_(x), is advantageously used as the adhesionlayer. The adhesion layer 6 may be thermally evaporated oralternatively, for example, produced by plasma polymerization fromsiloxane monomers, for example, TMS or HMDSO.

In a further intermediate step represented in FIG. 1C, an organic layer1, which preferably has a thickness of between 150 nm and 300 nm, isapplied onto the adhesion layer 6. The organic layer is advantageouslyformed from a thermally stable organic material. In particular, theorganic layer 1 may comprise melamine (2,4,6-triamino-1,3,5-triazine),MBP (5,5′-di(4-biphenylyl)-2,2′-bithiophene), TPD(N,N′-bis(3-methylphenyl)-N,N′-diphenyl-benzidine), NPB(N,N-di(naphth-1-yl)-N,N′-diphenyl-benzidine), TPB(N,N,N′,N′-tetraphenylbenzidine), TCTA(tris(4-carbazoyl-9-ylphenyl)amine), B2TP(5,5′-di-(4-byphenylyl)-2,2′-bithiophene), HMDSO (hexamethyldisiloxane)or parylene.

In the further intermediate step represented in FIG. 1D, a nanostructure11 was produced in the organic layer 1 by means of a plasma etchingprocess. During the plasma etching process, the organic layer 1 is, forexample, bombarded with ions by means of a plasma ion source in order toproduce the first nanostructure 11. Such a plasma etching process isknown per se from the documents DE 10241708 B4 or EP 2083991 B1 cited inthe introduction to the description, and will therefore not be explainedin further detail.

The nanostructure 11 advantageously comprises a multiplicity ofelevations and depressions, the depressions preferably extending intothe organic layer to a depth of at least 30 nm, in particular from about30 nm to 130 nm. Owing to the nanostructure 11, the effective refractiveindex in the organic layer 1 is advantageously lower than the refractiveindex of the organic layer 1 before the plasma etching process iscarried out. In particular, the nanostructured organic layer 1 has aneffective refractive index which is lower than the refractive index ofsilicone. The organic layer 1 provided with the nanostructure 11 may inparticular have a refractive index gradient, the refractive indexdecreasing in a direction extending away from the silicone surface 7.

In a further method step, which is represented in FIG. 1E, a cover layer4 is applied onto the nanostructure 11. The cover layer 4 may, inparticular, function as a protective layer for the nanostructure 11.Furthermore, the cover layer 4 may advantageously also lead to otherfunctionalities of the surface, and it may in particular function as anantifog layer or as a hydrophobic layer. The cover layer 4 isadvantageously no more than 40 nm thick, preferably between 10 nm and 40nm thick. Owing to the relatively small thickness, the cover layer 4follows the nanostructure 11 of the organic layer 1 essentiallyconformally. In particular, the nanostructure 11 is not planarized bythe cover layer 4. The cover layer 4 may comprise an inorganic layer, inparticular an oxide layer, for example, SiO₂ or Al₂O₃. The cover layer 4is preferably applied like the previously applied layers by a vacuummethod, so that advantageously all of the antireflection layer can beproduced by a vacuum process.

It is also conceivable for the cover layer 4 to be formed from aplurality of thin sublayers, the total thickness of the cover layeradvantageously being no more than 40 nm. The cover layer may, forexample, comprise an at least 10 nm thick SiO₂ layer, which impartspermanent antifog properties to the surface. Subsequently, for example,a polymer containing fluorine may be applied as a further sublayer ofthe cover layer, the polymer layer containing fluorine preferably havinga thickness of between 5 nm and 10 nm. The polymer layer containingfluorine advantageously leads to hydrophobization of the surface, sothat water can roll off the surface easily.

FIG. 2 represents the reflection as a function of the wavelength λ inthe visible spectral range for a film of a silicone which has beenprovided with an antireflection layer by a method according to FIGS. 1Ato 1E (Curve 21). For comparison, the reflection of the uncoatedsilicone film (Curve 22) is furthermore represented. In order to producethe antireflection layer on the silicone film, in a first step apretreatment by ion bombardment with argon ions and oxygen ions wascarried out for 100 seconds with an ion energy of 1 eV and a biasvoltage of 100 V in a vacuum evaporation deposition system of the typeAPS 904 (Leybold-Optics). Subsequently, a 10 nm thick SiO₂ layer wasdeposited as an adhesion layer. In a further step, a 150 nm thick layerof melamine was evaporation-deposited with a growth rate of 0.3 nm persecond.

A plasma etching process was subsequently carried out by means of aplasma ion source, in order to produce a nanostructure in the organiclayer of melamine. In order to produce the plasma, argon with a flowrate of 13 sccm and oxygen with a flow rate of 30 sccm were introducedinto the vacuum system. The plasma ion source was operated with a biasvoltage, which is a measure of the energy of the ions striking thesurface, of 120 V with a discharge current of 50 A. The plasma etchingprocess was carried out for 300 seconds, after which the thickness ofthe melamine layer is about 130 nm and the effective refractive index ofthe nanostructured melamine layer is about 1.1. A 20 nm thick layer ofSiO₂ was deposited as a cover layer on the nanostructure by electronbeam evaporation.

The average residual reflection in the wavelength range of from 400 nmto 800 nm is less than 0.5% for the silicone film provided with theantireflection layer under normal light incidence. The coated sample wasstored for 1000 hours at a temperature of 105° C., without the opticalproperties being modified. This illustrates the high thermal stabilityof the antireflection layer.

FIGS. 3A to 3F represent a variant of the method for producing anantireflection layer.

As represented in FIG. 3A, in the method an adhesion layer 6 and anorganic layer 1 are initially applied onto the silicone surface 7 of asubstrate 10, which may in particular be an optical element. As in thefirst exemplary embodiment, a pretreatment of the silicone surface byion bombardment may be carried out before the application of theadhesion layer 6.

In the intermediate step represented in FIG. 3B, a nanostructure 11 wasproduced in the organic layer 1 by means of a plasma etching process.

Unlike in the first exemplary embodiment, in a subsequent intermediatestep represented in FIG. 3C, a cover layer is not applied directly ontothe nanostructure 11, but instead an etch stop layer 3 which may inparticular comprise a silicon oxide, for example, SiO₂, is initiallyapplied. The etch stop layer 3 may, for example, have a thickness ofabout 15 nm.

In a further method step represented in FIG. 3D, a second organic layer2 is applied onto the first organic layer 1 provided with thenanostructure 11 and the etch stop layer 3.

As represented in FIG. 3E, in a further method step a secondnanostructure 12 is produced in the second organic layer 2 by means of asecond plasma etching process.

In the method step represented in FIG. 3F, a cover layer 4 was appliedonto the second nanostructure 12, in which case the cover layer 4 mayhave the same configurations and advantageous properties as in the firstexemplary embodiment.

The second exemplary embodiment therefore differs from the firstexemplary embodiment in that the antireflection layer comprises twoorganic layers 1, 2 arranged above one another, which are respectivelyprovided with nanostructures 11, 12 by a plasma etching process. In thisway, a particularly good antireflection effect can be achieved.Advantageously, the organic material of the second organic layer 2 has ahigher etching rate when the second plasma etching process is carriedout than the material of the first organic layer 1 in the first plasmaetching process. In this way, in particular, it is possible to producein the second organic layer 2 a nanostructure 12 which has an even lowereffective refractive index than the first organic layer 1 provided withthe nanostructure 11. Both the first organic layer 1 and the secondorganic layer 2 may respectively have a refractive index gradient, therefractive index decreasing in a direction extending away from thesilicone surface 7.

The second organic layer 2, like the first organic layer 1, isadvantageously formed from a thermally stable organic material. Inparticular, the organic materials mentioned in connection with the firstexemplary embodiment are suitable for this. As in the first exemplaryembodiment, the cover layer 4 may in particular be used to protect thenanostructure 12 and/or it may have antifog properties or hydrophobicproperties.

According to the method steps schematically represented in FIGS. 3A to3F, by way of example an injection-molded optical element made ofsilicone, which has a refractive index n=1.42 at λ=500 nm, was providedwith the antireflection layer. In this case, a plasma pretreatment wasinitially carried out in a vacuum evaporation deposition system of thetype APS 904 (Leybold-Optics), the silicone surface being bombarded for200 seconds with ions having an ion energy of at most 80 eV (set by abias voltage of 80 V).

After the plasma pretreatment, an approximately 150 nm thick organiclayer of MBP was applied by evaporation. The layer thickness was in thiscase measured by an oscillating quartz measuring system, with which themass increase is detected. By means of a plasma ion source, ananostructure was subsequently produced in the organic material MBP.Before the plasma etching process was carried out, in this exemplaryembodiment a 2 nm thin layer of TiO₂ was initially deposited and theplasma etching process was subsequently carried out by means of anargon/oxygen plasma. The plasma etching process was operated with a biasvoltage, which is a measure of the energy of the ions striking thesurface, of 120 V and a discharge current of 50 A. In this way, ananostructure with a depth of 80 nm and an effective refractive index ofabout 1.33 was achieved after an etching time of 350 seconds.

In a further step, an etch stop layer, namely a 15 nm thick SiO₂ layer,was evaporation-deposited. In a further step, in the same vacuumprocess, deposition of a second organic layer, namely a 250 nm thickmelamine layer, was subsequently carried out by thermal evaporation witha growth rate of 0.3 nm/s. A second plasma etching process wassubsequently carried out in order to produce a nanostructure in thesecond organic layer of melamine. This was done with a lower ion energyof 80 eV. Under these conditions, an etching rate of about 0.8 nm/s wasachieved in the organic layer of melamine, while under these conditionsonly a thickness reduction of less than 0.1 nm/s would occur in theunderlying MBP layer. After a plasma etching time of 150 s, thethickness of the melamine layer is still about 120 nm, the effectiverefractive index being about 1.1.

A cover layer, namely a 20 nm thick SiO₂ layer, was subsequently appliedonto the second nanostructure.

FIG. 4 represents the reflection R of the silicone surface provided inthis way with an antireflection layer in the wavelength range of from300 nm to 1200 nm for an angle of incidence of 0° (Curve 41) and anangle of incidence of 45° (Curve 42). For comparison, the reflection Rof the uncoated silicone surface for the angle of incidence 0° (Curve43) and the angle of incidence 45° (Curve 44) are represented. Theaverage residual reflection in the range of from 300 nm to 1200 nm forthe silicone surface provided with the antireflection layer is only lessthan 0.5% for normal incidence and less than 1% for the light angle ofincidence 45°.

The invention is not limited to the description with the aid of theexemplary embodiments. Rather, the invention covers any new feature andany combination of features, which includes in particular anycombination of features in the patent claims, even if this feature orthis combination is not explicitly indicated per se in the patent claimsor exemplary embodiments.

What is claimed is:
 1. An optical element comprising: a substrate havinga silicone surface; an antireflection layer overlying the siliconesurface, wherein the antireflection layer comprises a first organiclayer having a reflection-reducing nanostructure, the nanostructurehaving a depth of at least 30 nm; and a cover layer overlying the firstorganic layer, the cover layer having a thickness of no more than 40 nm.2. The optical element according to claim 1, further comprising anadhesion layer between the silicone surface and the first organic layer.3. The optical element according to claim 2, wherein the adhesion layercomprises a silicon oxide.
 4. The optical element according to claim 2,wherein the adhesion layer has a thickness of no more than 50 nm.
 5. Theoptical element according to claim 1, wherein the first organic layercomprises a material selected from the group consisting of: melamine(2,4,6-triamino-1,3,5-triazine), MBP(5,5′-di(4-biphenylyl)-2,2′-bithiophene), TPD(N,N′-bis(3-methylphenyl)-N,N′-diphenyl-benzidine), NPB(N,N-di(naphth-1-yl)-N,N′-diphenyl-benzidine), TPB(N,N,N′,N′-tetraphenylbenzidine), TCTA(tris(4-carbazoyl-9-ylphenyl)amine), B2TP(5,5′-di-(4-byphenylyl)-2,2′-bithiophene) and parylene.
 6. The opticalelement according to claim 1, further comprising a second organic layerdisposed between the first organic layer and the cover layer, whereinthe second organic layer comprises a second nanostructure.
 7. Theoptical element according to claim 6, wherein the second organic layerhas a lower effective refractive index than the first organic layer. 8.The optical element according to claim 6, wherein the first organiclayer and the second organic layer respectively have a refractive indexgradient, the refractive index gradient decreasing in a directionextending away from the silicone surface.